It has recently become possible to manufacture a high tenacity, high initial modulus fiber having a tenacity not less than 20 g/d and an initial modulus of elasticity not less than 500 g/d by the so-called liquid crystal spinning technique, wherein a polymer having a rigid molecular chain, for example, polyparaphenylene terephthalamide (hereinafter referred to briefly as PPTA) is dissolved in a suitable solvent, such as sulfuric acid, to a concentration at which the resulting solution shows the properties of a liquid crystal, and this solution is extruded through a spinneret. Such methods of manufacture of PPTA fibers are already in the stage of commercial implementation. However, such fibers have the disadvantage that in material cost as well as in production cost, they are by far more costly than ordinary fibers.
Meanwhile, the technique of producing a high tenacity, high initial modulus fiber from a flexible high molecular polymer has also been developed and is gathering much attention. This technique is known as the gel spinning method, by which polyethylene fibers having a tenacity at least about twice that of PPTA fiber and an initial elastic modulus approaching to its ultimate have been produced However, these fibers have the drawback of insufficient heat resistance because of their low melting properties.
Among the universal types of fibers, PVA fiber is excellent in tenacity and initial modulus of elasticity and, even in heat resistance, superior to polyethylene fiber. Therefore, it could be expected that if a technique were developed to produce a PVA fiber comparable to a PPTA fiber in tenacity and initial modulus of elasticity, this would represent a major contribution to this art, particularly in terms of reduced cost of manufacture and would lead to an expansion of uses.
Heretofore, various approaches have been explored to improve the tenacity and initial modulus of PVA fiber. For example, a method using a superhigh polymerization degree PVA having a molecular weight in excess of 500,000 is proposed in U.S. Pat. No. 4,440,711. However, this approach has the drawback that such a superhigh polymerization degree PVA is hardly available from commercial sources. Moreover, a PVA having such a superhigh degree of polymerization is only sparingly soluble in solvents and since solutions thereof are so high in viscosity, they are poor in spinnability. Therefore, it is inevitable to use low solution concentrations and this detracts from manufacturability.
In U.S. Pat. No. 4,603,083, it is disclosed that a PVA fiber having a tenacity of 19.6 g/d and an initial elastic modulus of 445 g/d could be manufactured by dissolving a high molecular weight PVA having a degree of polymerization of 4,000 in dimethyl sulfoxide (DMSO) to prepare a spinning dope and subjecting the resulting solution to dry-wet spinning. However, verification experiments made by the present inventors revealed that when the spinning dope is prepared using DMSO as a solvent, the stability of the dope is poor and it was difficult to manufacture a highly stretchable filament stably and continuously. Moreover, the crystalline heat of fusion of the fiber obtainable by drawing such filaments is as low as about 20 cal/g.
Japanese Patent Application (OPI) Nos. 108711/86 and 108712/86 (the term "OPI" as used herein refers to a "published unexamined Japanese Patent Application") propose the technique of extruding a spinning solution of a PVA having a polymerization degree of at least 1,500 in a nonvolatile solvent, such as ethylene glycol, glycerin or the like, in a coagulation solvent immiscible with the spinning solution, such as decalin, trichloroethylene or the like, by the wet or dry-wet spinning method. However, in these processes, the spinning speed is 5 m/min. at best and the required extraction of the nonvolatile solvent is so time-consuming that the technique cannot be successfully implemented on a commercial scale. Furthermore, this technique fails to accomplish an improvement in crystalline heat of fusion in any substantial degree, although it does improve the tenacity and initial elastic modulus of the fiber.
In the spinning processes disclosed in Japanese Patent Application (OPI) No. 85013/87, a mixture of water and DMSO, with addition of boric acid, is used as a solvent for PVA, but neither of the specifications includes references to the spinning stretch ratio which constitutes a feature of the present invention.
The method proposed in Japanese Patent Application (OPI) No. 90308/87, which comprises preparing a spinning dope by dissolving a PVA having a weight average molecular weight of 1.15.times.10.sup.5 in DMSO or water and extruding the resulting dope in methanol, is analogous to the method described in U.S. Pat. No. 4,603,083 referred to hereinbefore and, of course, has the same drawbacks.
While a variety of methods have been proposed for the manufacture of a high tenacity, high initial modulus PVA fiber as mentioned above, the spinning method using a PVA having a superhigh degree of polymerization is disadvantageous in that such a polymer is not readily available on the market and is expensive.
The dry-wet spinning method using DMSO as a solvent for PVA does not assure the stability of the spinning dope and hence fails to permit the continuous stable production of highly stretchable filaments. Moreover, the PVA fiber obtainable by drawing such filaments is low in crystalline heat of fusion
On the other hand, the wet or dry-wet spinning method comprising the extrusion of a solution of PVA in glycerin into a solvent such as decalin necessitates a low spinning speed which detracts from the commercial implementation of the method.